I am 71 and do not feel it. I still get up and down just fine and am still strong and sharp. Am I as good as I was when I was 30 to 40, NO. But I also know how to work smarter. I can turn down engine swaps and transmission swaps and heater cores, if I want to. I still do things like timing belts and chains. I don't want stuff that ties up my driveway for more than 2 days. An older small block or big block head gasket job, no problem. The only thing that is keeping me out of that 4 day a week tech job is moving my tools. I know I could still turn wrenches all day or at least most of the day because I have done it at home on some bigger jobs and this shop is not where I would be flagging hours like crazy. BUT, if I could find an advisor/writer job and it was the right fit, I would take it.
At 71 I am what they call, Full Retirement Age. I can work all I want and I get my full social security, even if I made $100k. Anything that is on the books up to $14k, I don't have to report or file income taxes at end of year. Over that and up, that is on the books/paycheck, up to $22K, I would pay 15% income tax. Any over that and I would be obligated to 18% income tax. I have a tax guy in KC, Mo., that I went to high school with and has been doing my taxes for 45+ years.
The work as a tech in his shop guy, was willing to work with me.